1. Field of the Invention
This invention is directed to a process for the alkylation of aromatic compounds to produce alkyl aromatics; to a reactor useful in such a process; and to processes which affect turbulent intermingling of reactants and a reactor useful in such processes.
2. Description of Related Art
Efficient mass transfer, control of reactant temperature, and co-production of energy at a sufficient level to be useful are several of the problems confronting today's chemical processing art. In many processes, the use of corrosive reactants and catalysts require more durable, stronger apparatuses, often made from expensive corrosion-resistant materials. For example, in various prior art processes for the catalytic alkylation of hydrocarbons, acid catalyst (e.g., hydrofluoric or sulfuric acid or aluminum chloride) is transported through the reaction system and through the recovery system. Apparatuses used in such processes (pumps, vessels, valves, heat exchangers, etc.) are made from expensive alloys. Temperature plays a critical role in such processes and can limit design possibilities.
The production of ethylbenzene by the traditional aluminum chloride (AlCl.sub.3) catalyzed reaction of ethylene and benzene has been in commercial use for decades. Ethylbenzene is used in large quantities for the manufacture of styrene monomer, the raw material for polystyrene. Often the catalyst is a liquid, AlCl.sub.3, complex. The reaction being carried out in a heterogeneous liquid medium composed of the catalyst complex and a mixture of ethylated benzenes. A two phase liquid product results: (1) the liquid AlCl.sub.3 complex (separated and recycled to the alkylator); and (2) a mixture of unreacted benzene and reaction products such as ethylbenzene, diethylbenzene and higher polyethylbenzenes.
The overall reaction can be expressed simply as: ##STR1## The actual chemistry involved is complex. First, there is the alkylation which involves the reaction of benzene and ethylene to form ethylbenzene as shown above. The alkylation is complicated by the occurrence of minor side reaction such as cracking and polymerization. However, of major importance is the formation of polyethylbenzenes: ##STR2## As illustrated by the equation above, the first alkyl group formed activates the aromatic nucleus so that the second alkylation proceeds more readily than the first and so on, at least until steric hindrance intervenes, although hexaethylbenzene is quite readily formed. This results in a mixture of mono, di, tri, and higher ethylbenzenes together with unreacted benzene.
Fortunately, the reaction is reversible, that is reversible in the sense that diethylbenzene, for example, will react under the influence of AlCl.sub.3 to form monoethylbenzene: ##STR3##
Thus, this transalkylation or reshuffling of ethyl groups among the aromatic rings takes a given reaction mixture to an equilibrium composition that is dependent only on the ratio of ethyl group to benzene rings present. It is these transalkylation reactions that permit virtually all the ethylene and benzene fed to the reactor system to be eventually converted to monoethylbenzene.
In one conventional ethylbenzene process, both the alkylation and transalkylation steps are carried-out in a single back-mix reactor using relatively long contact times and low temperatures (30-60 min. and 110.degree.-165.degree. C.) The conventional process also uses relatively high AlCl.sub.3 catalyst concentrations. For this reason, the catalyst must be separated from the reaction product and recycled to the reactor. U.S. Pat. No. 3,448,161 teaches the use of short contact times (2 min.) and temperatures up to 250.degree. C. U.S. Pat. No. 3,766,290 discloses the use of catalyst concentrations below the level that requires catalyst recycle. The advantages of separate alkylation and transalkylation reactors are revealed in U.S. Pat. No. 3,848,012. The concept of having separate alkylation and transalkylation steps, but combined in a single piece of equipment is claimed in U.S. Pat. No. 4,179,473. U.S. Pat. No. 3,501,536 teaches the use of a reactor with heat exchange tubes and a plurality of conduits that carry hydrocarbon reactants into a reactor space for intimate mixing and cooling of reactants and catalyst, the reactants entering through a series of spaced openings to the tubes which jet the reactants into an upward spiraling flow around the heat exchange tubes. U.S. Pat. No. 3,817,708 discloses a heat exchanger for use in an alkylation unit in which a heat transfer medium flows through U-tubes while an alkylation catalytic acid flows through the remaining space within a cylindrical shell that houses the U-tubes, the bends in the U-tubes approaching each other near the center of the shell in an adjacent, spaced-apart non-overlapping relationship. All of these processes have certain disadvantages which are overcome by the present invention.
Conventional alkylation reactors consist of open or baffled tanks or towers. These reactors are constructed of brick-lined steel or exotic alloys that resist corrosion by the acid catalyst. Tube or coil alkylation reactors have been disclosed in the literature, but have not been commercialized due to a variety of problems.
There has long been a need for an efficient process (and apparatus for such a process) for producing alkyl aromatics. There has long been a need for such a process in which not only is temperature controlled and optimized, but also from which useful heat energy may be efficiently extracted. There has long been a need for such processes and apparatuses permitting relatively high reaction temperatures. There has long been a need for such processes and apparatuses in which pressure is sufficient to maintain aromatic hydrocarbons in liquid phase. There has long been a need for an efficient process and for apparatus useful in it in which reactants are turbulently intermingled.